Vapor phase etching of oxide masked by resist or masking material

ABSTRACT

Hydrogen fluoride undercut of oxide layers may be reduced by using a low pressure mixture of gaseous hydrogen fluoride and gaseous ammonia mixture. Organic photoresists can be used as a masking material when using the gaseous hydrogen fluoride/ammonia mixture without resulting in an enhanced reaction rate. In addition, because of the reaction conditions, the dimensions in the oxide layer being etched can be specifically sized smaller than openings made in the overcoating masking material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to the two concurrently filed patentapplications having Docket No FI9-97-073 and FI9-97-074 and bearing thetitles of "TRENCH SIDEWALL PATTERNED BY VAPOR PHASE ETCHING" and "OXIDELAYER PATTERNED BY VAPOR PHASE ETCHING, respectively and the completecontents of these two concurrently filed applications is hereinincorporated by reference.

DESCRIPTION BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to computer chip manufacture,and more particularly to etch techniques used in constructing integratedcircuit structures.

2. Background Description

In the early days of production of integrated circuits, patterned resistmasks were used in concert with liquid, aqueous etches to remove theunderlying, regions that were not masked with resist. There wasdeterioration of the lithographically defined dimension caused byundercut of the resist masks by the isotropic character of the liquidetch. For this reason, more expensive reactive ion etching has largelysupplanted the early method of liquid etching. However, verticalsurfaces cannot easily be patterned with reactive ion etching.

Sometimes it is necessary to mask liquid phase etching of silicondioxide or doped silicon dioxide. The most convenient way is to useresist to define the region to be etched. However, aqueous solutions ofhydrogen fluoride can attack the interface between the resist and oxideleading to undercut of the resist with the best process conditions andleading to complete removal and failure of the resist mask under theworst conditions, where capillary action draws the aqueous solutioncompletely under the resist. If the resist detaches, then all the waferscan be contaminated by resist particles. Etching with conventional,gaseous vapor hydrogen fluoride is also known to penetrate resist, andcan even lead to enhanced etching under the resist. The aqueous filmsformed on the surface in this type of gaseous etching are also subjectto capillary action at the resist/oxide interface.

Other well known problems associated with aqueous processing whenhydrophobic resist or silicon surfaces are exposed is the deposition ofparticles that are suspended in solutions and the residue left by dryingof droplets of solution. Both problems are associated with hydrophobicsurfaces that cause solutions to "bead up" and particles to beattracted. Also, the surface tension of the liquid can preventpenetration and etching of small masked features.

It is possible in many applications to use a gaseous reactive ionetching process to remove unmasked material. This process does not havethe resist adhesion, resist undercut, solution penetration, particlecontamination from solution, and dried contaminant problems associatedwith aqueous processing. However, reactive ion etch (RIE) is expensivebecause only one wafer at a time is processed, the substrates aresusceptible to unwanted damage, and tooling is complex. In addition,only horizontal surfaces which are parallel to the surface of the wafercan be patterned because the incident ions which are used to produceetching travel perpendicular to the wafer surface. Masked surfacesperpendicular to the wafer surface are not patterned. There are stillapplications where a vertical surface must be patterned, or where lowcost is a requirement. A new method that does not degrade lithographicdimensions and which can process several wafers simultaneously with lowcost would be advantageous.

A particularly important application of patterning of vertical oxidesurfaces is in formation of a deep trench for use as in capacitor inDynamic Random Access Memories. Particles or watermarks left behind onthe hydrophobic sidewalls of the "aqueous processed" trench can causefailure of the trench. It is not possible to use Reactive Ion Etching topattern the vertical trench surfaces.

Vapor phase hydrogen fluoride exposures have used photoresist masks topattern silicon dioxide. However, conventional anhydrous vapor hydrogenfluoride, hydrogen fluoride/H₂ O and hydrogen fluoride/alcohol reactionsact through an intermediary film of liquid. Thus, they are alsosusceptible to loss of resist adhesion and loss of pattern integritythrough capillary action. In addition, the conventional processes takeplace with relatively high hydrogen fluoride partial pressures of tensof Torr or more. At these relatively high pressures, the hydrogenfluoride can even penetrate the resist layers and preferentially etchthe oxide under the masked areas. See, for example, Whidden et al., J.Electro Chem. Soc. 142, 1199 (1995). Furthermore, both the aqueous HFand standard vapor HF process isotropically etch the unmasked region ofoxide. It is not possible to etch an oxide layer to produce an exposedregion of the underlayer with dimension smaller than the resist opening,or with a mouth opening less than the resist opening and two times theoxide layer thickness.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved hydrogenfluoride based oxide etch technique to be used with masking materialsincluding organic photoresists.

It is another object to provide an improved method for forming a trenchfor use as in a capacitor in Dynamic Random Access Memories.

It is yet another object of this invention to provide a method forforming openings in an oxide layer on a substrate that are smaller thanthe openings in the overlying mask material.

According to the invention, a mask is positioned over an oxide layer tobe etched. The mask has one or more openings defining the pattern to beetched into the oxide. This invention allows the openings in the mask tobe larger in dimension than the openings to be formed in the oxidelayer. In addition, if desired, this invention allows the mask to becreated from an organic photoresist without enhancing the etch rate.After patterning the mask, the exposed oxide is reacted with gaseoushydrogen fluoride and ammonia. Finally, the mask and reaction productsare removed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of the preferredembodiments of the invention with reference to the drawings, in which:

FIG. 1 is a flow chart depicting the inventive method;

FIG. 2A is a cross sectional view of schematic of a substrate having anoxide layer and patterned mask;

FIG. 2B is a cross sectional view of a schematic of the substrate shownin FIG. 2A after etching the oxide layer by the inventive method;

FIG. 2C is a cross sectional view of a schematic of the substrate shownin FIG. 2A after etching the oxide layer by the prior art method; and

FIG. 3 is a schematic cross sectional view of the application of mask tomask vertical surfaces of deep trenches.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, a flowdiagram shows the steps of the invention. As can be seen in the stepshown in block 1, an oxide layer is formed on a substrate. The oxidelayer may be formed by thermal oxidation or deposition. Then, as shownin block 2, the oxide is coated with a mask material and, as shown inblock 3, the mask is patterned. Patterning of mask may be accomplishedby conventional methods including photolithography. The next step, asshown in block 4, is to react exposed oxide with hydrogen fluoride. Toaccomplish this, an individual substrate or a multiplicity of substrateswith similar patterns are placed in a chamber, preferably at roomtemperature, and then exposed to a mixture of hydrogen fluoride andammonia gas. The conditions in the chamber are preferably a totalpressure of below 30 mTorr and relative flow rates of 2:1 for hydrogrenfluoride:ammonia, respectively, for 40 minutes. The pressure is mostpreferably 7 mTorr.

A reaction product is formed as a result of the substrates' exposure tohydrogen fluoride and ammonia gas. This reaction product includes etchedoxide and reactants and combinations thereof Finally, as shown is block5, the reaction product is removed. Preferably, removal is accomplishedby heating the substrate to 100° C. to evaporate the reaction product asdescribed in U.S. Pat. No. 5,282,925 to Jeng et al., which is hereinincorporated by reference. Since neither the reaction product nor thereactants form liquids on the surface, capillary action is not possible.The reaction product may also be removed by rinsing.

The steps of the invention are illustrated in the schematic crosssectional views shown in FIGS. 2A and 2B. In FIG. 2A, there is shown asubstrate 11 having a silicon dioxide layer 12 deposited thereon. Thesilicon dioxide layer 12 may for example have a thickness between 10-600Å. A patterned mask 13 is deposited on the silicon dioxide layer 12. Asshown, the mask 13 has an opening 14 which exposes portions of theunderlying silicon dioxide 12. While any masking material can be usedfor patterning the underlying silicon dioxide, it has been discoveredthat this invention can be practiced with organic photoresists (e.g.,JSR 785 and AZ7500) which are available from a number of commercialvendors including the Shipley Company. This was unexpected becausetypical aqueous hydrogen fluoride (HF) etches lift the edges ofphotoresists, thus degrading the desired dimension. By contrast, withthis invention, it is possible to improve upon the defined dimension.

FIG. 2B shows the substrate after an opening 15 is etched in the silicondioxide layer 12. Hydrogen fluoride does not penetrate the mask 13because the operating pressure is about 7 mTorr. This pressure is morethan three orders of magnitude lower than in standard vapor HFprocesses. Diffusion of HF through the mask is very slow and any waterproduced by the reaction escapes without forming a liquid on thesurface. Loss of mask adhesion due to capillary action drawing thesolution under the mask does not occur because the adsorbed film in theammonia/hydrogen fluoride etching reaction more resembles solid ammoniumbifluoride, rather than a conventional liquid that forms in vaporhydrogen fluoride processes or in aqueous etching. In fact, when theoxide thickness is near 150 Å, for instance, the dimension of theopening 15 is smaller than the opening 14 in the mask and is muchsmaller than the contour of the opening with an aqueous process 24 as isshown in FIG. 2C. The opening 15 has diagonally sloped sides 16 whichnarrow toward the surface of the substrate 11.

Returning the FIG. 2B, the undercut 17 with the inventive method issmall because the reaction is not isotropic. The solid reaction product(not shown) occupies more volume than the etched oxide. It expands fromthe undercut and overlaps with reaction product from region 18. As thereaction product forms and expands, the reaction rate on the productcovered surfaces declines. After about 600 Å of reaction product haveformed (corresponding to 200 Å of reacted oxide), the reaction stops.The decline in reaction rate and termination occurs sooner in regions ofreaction product overlap than where there is no overlap. Because of theoverlap of product from undercut 17 under the overhang of the mask withproduct from above region 18, termination of the reaction leaves acontact area 15 that is smaller than the dimension of opening 14 in themask 13 by the amount bracketed by region 18. Preferably, the oxidelayer 12 that is etched is somewhat less than 200 Å in thickness. Region18 is smaller than ˜400 Å and region 17 is smaller than 200 Å. Atdistances greater than 400 Å from the mask edge, there is no overlap ofreaction product with reaction product from under the mask overhang 17.

Currently, the smallest opening that can be lithographically definedwith conventional photolithography is about 1800Å. Addition of thisinvention reduces the defined dimension by 400 Å on each side to producea 1000 Å opening in the oxide. Thus, contact openings 15 of 1000 Å to1800 Å in oxide layers can be defined by this process using conventionalphotolithography to pattern a photoresist layer used as mask 13. Asphotolithography processes improve, the contact opening should proceedto dimensions smaller than 1000 Å. For example, it is expected thatusing the procedure of this invention, one will be able to reliablypattern contact openings from 10 Å to 1800 Å.

In FIG. 2C there is shown a cross sectional view of substrate 21 similarto that shown in FIGS. 2A and 2B. The substrate 21 has oxide 22 and mask23 positioned on its surface. As shown by illustration in FIG. 2C, theoxide 22 has been etched by aqueous hydrogen fluoride. Etching withaqueous hydrogen fluoride leaves an overhang 26 equal to the thicknessof the oxide layer 22 as shown by contour 24. This is because thereaction occurs at the same rate on all exposed surfaces (i.e., it isisotropic). Reaction termination does not occur for this reaction as itdoes in the present invention. If there is loss of mask adhesion whenetching with aqueous hydrogen fluoride, then the overhang 26 would beeven greater. Thus, unlike the present invention, the cleared region 25cannot be smaller than the opening 27 in the mask 23, and can be muchlarger.

The reduced attack of the mask/oxide interface of the present inventionstems from several features of the gaseous ammonia/hydrogen fluoridereaction. The principle feature is that no liquid is involved. Asmentioned previously, aqueous hydrogen fluoride can be drawn along themask/oxide interface by capillary action. Even commercially availablegaseous hydrogen fluoride and hydrogen fluoride/water or hydrogenfluoride/alcohol reactions can be subject to capillary effects. In thesereactions the alcohol or the water that is added or that is availablefrom the reaction products is used at a pressure that forms a thinadsorbed film of the liquid on the surface. The adsorbed film could besubject to capillary effects. The ammonia/hydrogen fluoride reactionalso forms an adsorbed film, but the film is a solid ammonium bifluorideinstead of a liquid. The solid is not mobile or subject to capillaryaction. A second feature is formation of a solid reaction product whichoccupies more volume than the volume of oxide etched and which blocksthe interface from attack. A third feature is that the solid productinhibits diffusion of the reactant to the surface of the oxide. Awayfrom an interface, reactant is able to diffuse to the surface from alldirections. Thus, the reaction rate is higher away from a masked regionthan at the edge where reactant can only diffuse in from one direction.This effect contributes to enabling oxide openings to be smaller thanthe mask openings (it being expected that the oxide opening will be2-800 Å smaller in dimension than the mask opening).

A particularly useful application of masking of silicon dioxide is information of the buried plate of the deep trench storage capacitor usedin dynamic random access memories. A requirement of this application isthe removal of oxide on a surface perpendicular to the substratesurface. This application is shown in FIG. 3. As can be seen in FIG. 3,a trench 31 is formed in a silicon substrate 32, the trench 31 is linedwith arsenic doped glass 33 and filled with masking material 34. Otherinsulator materials such as silicon dioxide could be used as the liner33, and other substrate materials besides silicon could be used. Themasking material 34 is recessed in the trench 31 with respect to the topsurface of the substrate by any suitable means including being depositedat a volume that makes it recessed in the substrate 32. The liner 32 isthen stripped from the vertical sidewalls of the upper portion 35 of thetrench 31 with gaseous hydrogen fluoride and ammonia at low pressure asdescribed in detail above. After gaseous hydrogen flouride:ammoniaetching, the remaining liner can then be used to dope the trench. Itshould be understood that a wide variety of other dopants can be usedfor this and other applications such as phosphorous, boron, indium, andantimony. Because this process patterns a vertical surface, RIE couldnot be used for this application (i.e., RIE works by directionalbombardment, thus it can only be used to pattern features that areperpendicular to the direction of bombardment). Thus, in thisapplication, the invention can include formation of a vertical surfacein the substrate prior to the first step shown in block 1 of FIG. 1. Thevertical surface can be part of the trench structure.

With this invention, the substrate can include hydrophobic features and,particularly, hydrophobic features of small dimension. The hydrophobicfeatures can include silicon surfaces. The hydrophobic surface is only aproblem when a liquid is in contact with the hydrophobic surface. Thesubstrate can include features to be formed at a dimension smaller thanthe lithographically defined dimension. Etching oxide with a gaseousmixture of hydrogen fluoride and ammonia solves problems relating tomasks, and is particularly useful for recessing arsenic glass whenforming a deep trench. Less attack of the mask/oxide interface occurswhen etching is with the gaseous process. In addition, since the processis gaseous, no watermarks or suspended particles are attracted to thehydrophobic surface of the silicon that remains after removal of thearsenic glass because a liquid is not used.

While the invention has been described in terms of its preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

We claim:
 1. A method of patterning oxide for an integrated circuit,comprising the steps of:depositing an organic photoresist on an oxidelayer to be patterned, the oxide layer being positioned on a substrate;photolithographically patterning an opening in the organic photoresistto form first and second edges which define an exposed portion of theoxide layer; etching the exposed portion of the oxide layer with neutralmolecules from a gaseous hydrogen fluoride/ammonia mixture whilesimultaneously preserving the organic photoresist such that the organicphotoresist at the opening remains in substantial contact with the oxidelayer subsequent to said etching; producing a reaction product from saidetching step, the reaction product being a solid that occupies morevolume than the etched oxide, wherein the organic photoresist forces thereaction product from under the edges of the organic photoresist intothe exposed portion of the oxide layer, thereby slowing the etching nearthe edge; and removing the reaction product formed in said etching step.2. The method of claim 1 wherein said removing step is performed byheating said reaction product to at least 100° C.
 3. The method of claim1 wherein said etching step is performed in an etch chamber operatingunder the conditions of a total pressure of less than 30 mTorr.
 4. Themethod of claim 3 wherein said total pressure is approximately 7 mTorr.5. The method according to claim 1, wherein said etching step produces alateral dimension of said photoresist not in contact with said oxidethat is smaller than a thickness of said oxide.
 6. The method accordingto claim 1, wherein said reaction product solid includes silicon fromsaid etched oxide.
 7. A method of patterning oxide for an integratedcircuit, comprising the steps of:positioning a mask on an oxide layer tobe patterned, the oxide layer being positioned on a substrate, the maskhaving first and second edges which define a first opening of a firstdimension which forms an exposed portion of the oxide layer; etching theexposed portion of the oxide layer with neutral molecules from a gaseoushydrogen fluoride/ammonia mixture while simultaneously preserving themask such that the mask at the first opening remains in substantialcontact with the oxide layer subsequent to said etching; producing areaction product from said etching step, the reaction product being asolid that occupies more volume than the etched oxide, wherein the maskforces the reaction product from under the edges of the mask and intothe exposed portion of the oxide layer, thereby slowing said etchingnear the edge; and removing the reaction product formed in said etchingstep, said etching and removing steps being performed under conditionswhere a second opening of a second dimension is formed, the secondopening residing in the oxide layer and contacting the substrate, thesecond dimension of the second opening being smaller than the firstdimension of the first opening.
 8. The method of claim 7 wherein saidremoving step is performed by heating said reaction product to at least100° C.
 9. The method of claim 7 wherein said etching step is performedin an etch chamber operating under the conditions of a total pressure ofless than 30 mTorr.
 10. The method of claim 9 wherein said totalpressure is approximately 7 mTorr.
 11. The method according to claim 7,wherein said etching step produces a lateral dimension of saidphotoresist not in contact with said oxide that is smaller than athickness of said oxide.
 12. The method according to claim 5, whereinsaid reaction product solid includes silicon from said etched oxide.